I'm looking for the quote/quotes in which Plato regards the disabled people as if they were "non-human." And - how did he justify his view?
The passage you are looking for is Republic, V. 460C: ‘removed from sight into some secret and hidden place’ (T. Griffith, Plato: The Republic, Cambridge: CUP, 2000: 158). The passage is odd in at least one respect. As Patterson comments: ‘Infanticide or exposure seems to be the intent, but why the cryptic language?’ (Cynthia Patterson , ‘"Not Worth the Rearing": The Causes of Infant Exposure in Ancient Greece’, Transactions of the American Philological Association (1974-2014), Vol. 115 (1985), pp. 103-123: 113.) It is not as if Plato were referring to some totally anomalous social innovation. Both practices were widespread in Ancient Greece. Strictly ‘infanticide’ – kill a child (paidoktoneo) and ‘exposure’ – leave a child without nutriment, shelter or protection (ektithemi) are not equivalent terms: the first implies a direct intention to kill the child, the second only foresees death as a probable result. In practice and in context, of course, they amount to the same thing.
But just who are to be removed in this way?
1.The offspring of inferior guardians
Plato makes clear that not only in the interests in controlling the population size of the city but also in order to preserve the guardian class in a state of top excellence, sexual relations among the guardians are to be carefully regulated. All guardians are among the finest of humanity; they are ‘children of gold’ (R. III.414c-415d; Griffith: 107-8) but not all gold is of the same purity and excellence. There are ‘inferior guardians’. They are not to be demoted to one of the other two classes in the city but they are near the border of acceptability. On no account should their offspring be preserved among the guardian class; the inferior may produce the yet more inferior. (R.V 460C.)
2.The maimed or mutilated
These offspring (of the inferior guardians) should be ‘removed from sight into some secret and hidden place’ – but not only such offspring. If by chance even the best among the guardians to anaperon (something maimed or mutilated) is born, it must share the same fate (R. V.460c).
3.The offspring of guardians outside the age-limits
Age-limits are set for optimal procreation: 20 – 40 for women, and for men 25 – 55 (R.V.460E; Griffith: 159: if, following Laws VI.772D rather than VI.721A, VI.785B, we take 25 as the age ‘when his days as a sprinter are behind him’). When elderly guardians, presumed safe from procreation, are allowed to engage in sexual activity Plato is aware that reproduction might result. ‘If there is a pregnancy, then ideally the embryo should never see the light of day’ (R.V.461C; Griffith: 159). The language here suggests abortion as does Shorey’s rendering of the phrase: ‘if they are unable to prevent a birth to dispose of it’ (Paul Shorey, Plato, Republic, I, Harvard: Loeb Library, 1930/ 1994: 467).
No lost humanity
I might add that it does not follow that the offspring who fall within these three categories are non-human [see textbox above], nor does Plato say or imply that they are. His idea is that as defective – human but defective – they are not worth rearing. This is harsh but not literally dehumanising.
I can't vouch for the source, but here's one quote...
This is the kind of medical provision you should legislate… provide treatment for those of your citizens whose physical constitution is good. As for the others, it will be best to leave the unhealthy to die…
According to the article, "Plato wished to rid the population of people with disabilities because he felt that they did not have a quality of life associated with human dignity."
I love the way he compares this philosophical view with Hollywood.